The purpose of these investigations is to learn more about the role of PMN and mononuclear leukocytes in the pathogenesis of periodontal disease. These cells are conspicuous elements in inflamed gingival exudates and tissues and their interactions with plaque bacteria and with various host cells is probably pertinent to the disease process. We are monitoring several aspects of leukocyte function in different models of human periodontal disease, including experimental gingivitis, adult periodontitis and juvenile periodontitis. We want to ascertain whether leukocyte function is altered in these patients when compared to normal persons. We have embarked upon a concentrated effort to define the mechanisms and consequences of plaque bacteria interaction with inflammatory cells; most recently we have identified Actinobacillus actinomycetemcomitans as a plaque organism with leukotoxic potential. This organism is found with relatively high frequency in juvenile periodontitis, and we believe it may kill leukocytes in the gingival crevice. Lastly, we are engaged in researches on the effects of PMNs or PMN products on lymphocyte responses in periodontal disease. There is reason to believe that this type of interaction may conceivably modulate the immune response in the gingival environment.